A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus it is desirable to know characteristics of the projection lens used for imaging the pattern on the patterning device onto the substrate. Such characteristics may also be referred to as properties or parameters of the projection lens. One such property is the numerical aperture (NA) of the lens which affects the imaging of the lithographic apparatus. Knowledge of the exact value of the numerical aperture can be used in simulations to determine settings and process windows for the lithographic apparatus. In some apparatus, the projection lens has an adjustable numerical aperture which is defined by elements such as an adjustable diaphragm at a pupil plane in the projection lens system. Measurement of the actual numerical aperture setting is thus performed.
Previously, the numerical aperture has been measured by imaging defocused pinholes on to a resist-coated substrate. The defocusing is performed by placing the pinholes on top of a mask (or using a mask upside down, such that the opaque layer defining the pinholes is displaced from the usual plane of the patterning device. Diffractive features, such as gratings or arrays, are provided inside the pinholes so that the radiation fills the complete numerical aperture of the projection lens system. However, this technique has the problem that extensive measurement analysis of the resist is necessary, which is slow, and the result is not a simple direct measurement performed on the apparatus.
Another characteristic of a lithographic apparatus is the telecentricity of the projection lens and also of the illuminator which provides the radiation beam for the patterning device and projection lens. Non-telecentricity of the illuminator and projection lens can cause overlay problems. The non-telecentricity of the projection lens affects the imaging performance. Previously the telecentricity has been measured quantitatively by performing overlay measurements at different focus levels of the substrate. However, this method also suffers from the problems of being slow and cumbersome, and the method is not very sensitive to telecentricity of the projection lens.